In this study we propose to examine the hypothesis that fearlike responses to the electrical stimulation of the dorsal midbrain tegmentum (DMT) in rats is sensitive to brain serotonin manipulations and is potentially useful as an animal model of human anxiety. Bipolar stimulating electrodes will be implanted bilaterally into the dorsal midbrain tegmentum of rats. The aversive nature of each electrode will be behaviorally verified by determining its efficacy in eliciting bar- pressing for escape. The animals will next be trained to bar-press in a decremental escape format. In this format each bar-press will decrement the stimulation current by a predetermined fraction of the initial current level. The primary factor to be examined will be the effects of serotonin-modifying drugs on decremental bar-pressing behavior. These will include the serotonin-depleting drugs, parachlorophenylalanine and 6-flurotryptophan; the serotonin precursor, 5-hydroxytryptophan; and the serotonin analogue, lysergic acid diethylamide. To examine further whether the DMT-stimulated rat is a useful model for anxiety, the effects of phenothiazine and benzodiazepine tranquilizers upon the decremental escape bar-pressing will be examined. Brain levels of biogenic amines will be determined by biochemical assays. The location of each stimulating electrode tip will be histologically verified. Important controls will include saline-injected animals and DMT-stimulated animals who demonstrate non-fearlike behavioral responses, such as circling behavior.